From the non-Canon Big Finish stories, Dilijan is an ancient Immortal who inadvertently assisted in the creation of the Four Horsemen, and who dedicated his existence to pursuing an addiction to the despair-laced Quickenings of other Immortals and a quest of vengeance against the Horsemen. In the early 21st century, Dilijan was cornered by Methos, who then turned his terrible experiments against him as an act of vengeance for Dilijan's murder of Methos' lover, Violetta. So much negative Quickening energy was driven into Dilijan that his mind was seared and destroyed, left so full of agony that he could not form new memories, and left as a useless, quivering hulk of misery condemned to live forever. Dilijan is last seen raving and sobbing in a mental institution. (see The Promise)
Born before recorded history, Dilijan was a leader among his people. His First Death occurred when he slipped on ice while retrieving wandering livestock, and suffered a fatal head injury. He created a band of Immortals who moved across the world improving the lives of mortals. At some point, Dilijan began to experiment with expanding his trepanning skills (acquired during his first mortal life) to the treatment of Immortals suffering from emotional and mental wounds.
Dilijan participated in the battle of Kamtal with Caspian against an Immortal army lead by the unbelievably ancient and powerful Immortal Karniel. Caspian beheaded Karniel with the assistance of an army comprised of mortal and Immortal warriors. Taking Karniel's head and Quickening filled him with Karniel's personality, and he lashed out, killing all in his path, until he left the battle. Dilijan lead Caspian's pursuers away, and doubled back to kill Caspian -- or so he claimed later. However, instead of killing Caspian, Dilijan performed his trepanning technique upon him. Instead of draining away Karniel's negative power, Dilijan absorbed Caspian's virtue and self-sacrifice -- the very traits that had driven Caspian to face Karniel in the first place. The end result was Caspian rendered a murderous maniac, while Dilijan felt himself empowered with an urge to serve his kind. Thus empowered, Dilijan began to create his order of acolytes and informers intending to continue his work.
Dilijan traveled with Silas for a time, and felt admiration and affection for him. After Silas' wife was beheaded fighting bandits, and her Quickening went to her husband, hearing her voice in his head all of the time drove Silas mad. Dilijan attempted to use his trepanning technique to ease Silas's pain. The procedure went horribly wrong, Dilijan's tool broke off in Silas's brain, rendering him an ultraviolent man-child. Dilijan eventually parted ways with Silas, but kept tabs on him via a network of loyal acolytes and manipulated vessels. Periodically, Dilijan would task his acolytes to conduct experiments on Silas, as Dilijan labored to repair the damage he had caused. Note: At one point in All the King's Horses, Dilijan implies that his first trepanning experiment was the one he performed on Caspian.
Kronos joined Dilijan's band of Immortals shortly after his own First Death. He had been slowly throttled to death by the Immortal leader of a band of raiders. As Kronos died, his last thoughts were regret that he had never enjoyed a killing the way his killer had savored his death. Dilijan welcomed Kronos as an equal and partner.
Dillian and Kronos traveled together for a time, and Dilijan explained some parts of Immortality to Kronos, but avoided revealing information about The Game or Quickenings, for fear that Kronos would find Quickenings and The Game too enticing to resist. Dilijan planned to inform him of the full spectrum of Immortality when he felt Kronos was ready. Then Dilijan's band came upon the tracks of the band of raiders that had killed Kronos. Dilijan agreed to hang back and allow Kronos to confront them.
Silas was among the raiders -- he sided with Kronos, and revealed the nature of The Game and Quickenings to Kronos. Silas and Kronos joined forces and returned to Diljan; together, they slaughtered Diljan's entire band.
Methos had joined the group only days before, and was still a non-entity to them. He banded together with Kronos and Silas, wiping out Dilijan's group. Kronos spared Dilijan, "killing" him, but not taking his head.
Fifty years after the union of the three Horsemen and the destruction of Dilijan's band, Kronos and the others, now including Caspian, found Dilijan again. Dilijan had formed a new tribe, this time of mortals. Kronos and the Horsemen wiped out the entire group, burned their homes and salted the ground. Before killing Dilijan, Kronos promised that he would find Dilijan and punish him again every fifty years.
Dilijan went to ground, and managed to avoid Kronos for several hundred years, build his network, expand his ministry, and further his work with manipulating Quickenings. He also contemplated vengeance.
Over the next couple of thousand years, he would engineer the fall of the Horsemen and perfect his Quickening-manipulation techniques. Years later, Dilijan engineered an Immortal called Kyne using his skills at manipulating Quickenings. He sent Kyne to the Horsemen with instructions to sow dissent and distrust among them, particularly regarding Kronos. Kronos grew distracted with suspicions and petty jealousies. Eventually, Kyne convinced them to risk a sea journey. Once the ship was out of sight of land, in the midst of a storm, Kyne climbed to the mast and beheaded himself. His Quickening ignited lamp oil stored in the ship's hold. The ensuing explosion destroyed the ship, and scattered the Horsemen.
Before their deaths, he would exact his personal vengeance on Kronos, Caspian, and Silas. Dilijan finally learned how to heal Silas, capturing him he performed microsurgery and removed the stone drill bit from his brain, restoring him to his former intelligence and coherence. Dilijan and Silas had a long discussion of Silas's history and Dilijan's research, which ended when Dilijan revealed that he was going to replace the drill bit and return Silas to a lumbering, violent hulk. When Silas demanded to know how DIlijan knew it would work, Dilijan revealed that he had done it before.
When it came to Caspian, Dilijan lured him with tales and legends of the "Pain Eater," an Immortal who could absorb the pain and suffering of other Immortals; in other words, Dilijan himself. When Caspian was within his power, he once again conducted his trepanning procedure, only this time it was a success. Caspian spent two months under Dilijan's watch recovering from his ordeal under Karniel's Quickening. Then Dilijan revealed Caspian's original procedure, as well as Dilijan's role in it.
As punishment for Caspian's actions against Dilijan's people, he was going to take away Caspian's peace and return Karniel's Quickening. Caspian begged Dilijan to kill him; Dilijan explained that he wanted to ruin Caspian, to make sure his sacrifice at Kamtal was not just forgotten, but utterly eclipsed by the enormity of Caspian's evil acts. Dilijan then not only returned Karniel's Quickening, but scoured Caspian of all positive remnants of his own Quickening and those he had taken from good and noble Immortals during his lifetime up to that point. Dilijan promised that when Caspian came close to triumphing over his darkness in future, Dilijan would take away the light again.
Kronos's punishment was less elaborate. Because Kronos deserved so much less than Dilijan's victims (sarcasm). After the destruction of the ship that essentially destroyed the Horsemen, Kronos came to be lying on a beach, half-drowned and bound tightly with ropes. Dilijan explained that he had sent Kyne to them, intending to destroy them. Furthermore, he would never see Kronos again, but Kronos would see his acolytes and servants. He would use them to ensure that Kronos was never able to create another brotherhood. Every time Kronos came close to creating a new band, his plans were laid to waste by Dilijan's acolytes and manipulated Immortals.
Methos's punishment was possibly the most elaborate. Dilijan created another manipulated Immortal, a man named Dorn. He sent Dorn to Methos with the impulse to challenge the Horseman and lose. Dilijan had engineered Dorn's Quickening as an antidote to Methos's murderous impulses. After taking Dorn's head, Methos began to be sickened by the violence of his life, and started to seek a better way of life. Note: in The Promise, Methos and Dilijan do not come to a conclusion regarding Methos's change of heart and Dorn's role in it.
Once Dorn's influence and Methos determination had turned Methos's life around, Dilijan bided his time before enacting the final portion of his plan. He waited until Methos was not just happy but in love, and then he struck. He slipped Methos' lover, Violetta, a slow-acting, agonizing poison. As Violetta lay dying, she made Methos promise not to kill the man behind her murder. She knew he would revert to being a killer (Methos had confessed his past to her), and the man he was then, the man she loved, would vanish. Methos promised, and waited over a hundred years to find Dilijan.
When he found Dilijan, they "compared notes," so to speak, and Methos burned out Dilijan's mind by channeling six of his "batteries" (Immortals he had hollowed out and filled with the dark Quickening energy) into him. He then abandoned Dilijan at a mental institution, a gibbering, mindless wreck.